The present invention relates to electrostatic chucks useful for holding substrates during processing.
Electrostatic chucks are used to hold semiconductor substrates, such as silicon wafers, in a process chamber. A typical electrostatic chuck comprises an electrode covered by a dielectric layer. In monopolar chucks, an attractive electrostatic force is generated when the electrode of the chuck is electrically biased by a voltage and an electrically charged plasma in the chamber induces electrostatic charge in the substrate. A bipolar chuck comprises bipolar electrodes that are electrically biased relative to one another to generate the electrostatic attractive force.
The electrostatic attractive force generated by electrostatic chucks can also be of different types. As schematically illustrated in FIG. 1a, a chuck 10a having a dielectric layer 11 with a high electrical resistance results in coulombic electrostatic forces where opposing electrostatic charges accumulate in the substrate 12 and in the electrode 13 of the chuck. The coulombic electrostatic force is described by the equation:   F  =            1      2        ⁢          xe2x80x83        ⁢          ϵ      0        ⁢                            ϵ          r                ⁡                  (                      V            t                    )                    2        ⁢    A  
where ∈0 and ∈r are the dielectric constant of vacuum and relative dielectric constant of the dielectric layer 11, respectively, V is the voltage applied to the electrode 13, A is the area of the electrode, and t is the thickness of the dielectric layer.
With reference to FIG. 1b, Johnsen-Rahbek electrostatic attraction forces occur in the chuck 10b when an interface 14 between a low resistance or leaky dielectric layer 15 and the substrate 12, has an interfacial contact resistance much greater than the resistance of the dielectric layer 15, i.e., when the resistance of the dielectric layer 15 is typically from about 1011 to about 1014 xcexa9/cm. Free electrostatic charge drifts through the dielectric layer 15 in the applied electric field, and accumulates at the interface of the dielectric layer 15 and the substrate 12. The charge accumulated at the interface generates a potential drop represented by the equation:   F  =            1      2        ⁢          xe2x80x83        ⁢                            ϵ          0                ⁡                  (                      V            δ                    )                    2        ⁢    A  
where xcex4 denotes the contact resistance of the air gap 14 between the substrate 12 and the low resistance dielectric layer 15. The Johnsen-Rahbek electrostatic attractive force is typically higher than that provided by coulombic forces, because polarization in the dielectric layer 15, and free charges accumulated at the interface 14 combine to enhance electrostatic force. This provides a stronger electrostatic force that more securely holds the substrate 12 onto the chuck and also improves thermal transfer rates at the interface. Also, the lower voltages used in these chucks reduce charge-up damage to active devices on the substrate 12.
The dielectric layers 11, 15 covering the electrode 13 of these chucks typically comprise a thin polymer film, such as polyimide, adhered to the electrode, as for example disclosed in U.S. Pat. No. 5,745,331, patent application Ser. No. 08/381,786, entitled xe2x80x9cElectrostatic Chuck with Conformal Insulator Film,xe2x80x9d filed on Jan. 31, 1995, to Shamouilian, et al., which is incorporated herein by reference. However, the substrate held on the chuck often breaks or chips to form fragments having sharp edges that puncture the polymer film and expose the electrode. Exposure of the electrode at even a single pinhole in the dielectric layer can cause arcing between the electrode and plasma, and require replacement of the entire chuck. Polymers also have a limited lifetime in erosive process environments, such as processes using oxygen-containing gases and plasmas. Also, polymers or adhesives used to bond the polymer films to the chuck often cannot operate at elevated temperatures exceeding 1000xc2x0 C.
Polycrystalline ceramics have also been used to form the dielectric layer to provide increased puncture resistance and higher temperature performance, as for example, described in U.S. Pat. No. 5,280,156 to Niori; Watanabe, et al., in xe2x80x9cRelationship between Electrical Resistivity and Electrostatic Force of Alumina Electrostatic Chuck,xe2x80x9d Jpn. J. Appl. Phys., Vol. 32, Part 1, No. 2, (1993); or xe2x80x9cResistivity and Microstructure of Alumina Ceramics Added with TiO2 Fired in Reducing Atmosphere,xe2x80x9d J. of the Am. Cer. Soc. of Japan Intl. Ed., Vol. 101, No. 10, pp. 1107-1114 (July 1993); all of which are incorporated herein by reference. The ceramic dielectric layers typically comprise a low conductivity polycrystalline ceramic, such as a mixture of Al2O3 and TiO2, or BaTiO3. However, polycrystalline ceramics such as Al2O3 doped with TiO2 often have an electrical resistance that changes with temperature, and can exhibit low or insufficient electrical resistance at high temperatures. Also, polycrystalline ceramics comprise small grains or crystals that typically have a diameter of 0.1 to 50 microns, and have grain boundaries containing a mixture of glassy materials that hold the grains together. When such ceramic layers are exposed to erosive environments, such as a fluorine containing plasma, the plasma etches away the grain boundary regions causing the ceramic grains to loosen and flake off during processing of the substrate. Abrasion of the substrate against the chuck can also cause ceramic grains to flake off the chuck. These particulate ceramic grains contaminate the substrate and/or process chamber and reduce the yields of integrated circuit chips from the substrate.
Dielectric layers comprising a thin wafer of monocrystalline ceramic that is made of a few, relatively large, ceramic crystals have also been used to cover the electrode. For example, U.S. Pat. No. 5,413,360 to Atari, et al., describes an electrostatic chuck consisting of a monocrystalline ceramic wafer covering an electrode on a dielectric plate. Atari teaches that a bonding agent, or a high temperature joining method, is used to join the monocrystalline ceramic wafer to the electrode of the chuck. In another example, U.S. Pat. No. 5,535,090 to Sherman, filed Mar. 3, 1994, discloses an electrostatic chuck comprising small segments of monocrystalline ceramic wafers adhered to the surface of an electrode using a high temperature vacuum braze with a suitable brazing alloy. For example, a platinum layer can be sputtered onto the monocrystalline ceramic layer and a platinum paste used to adhere the monocrystalline ceramic layer to the metal electrode.
One problem with such chucks arises from their structure, which typically comprises a single relatively thin monocrystalline ceramic wafer bonded to the metal electrode with a layer of bonding material therebetween, and supported by a metal or dielectric plate made from another material. During the bonding process or during use of the chuck in an erosive process environment, the thermal expansion mismatch between the monocrystalline ceramic wafer and the electrode can result in failure of the bond. Also, the bonding material is typically a metal based material that thermally or chemically degrades during use of the chuck in reactive processes, causing failure of the chuck and movement or misalignment of the substrate during processing. The thin monocrystalline ceramic wafer and electrode can also separate from the supporting dielectric or metal plate at high temperatures due to stresses arising from the thermal expansion coefficient mismatches. Another problem arises because grooves, channels, and other hollow spaces which are used to hold coolant or to supply helium gas to the interface below the substrate, are difficult to form in the brittle, hard, and thin monocrystalline ceramic layers. During the series of machining or drilling steps that are used to form these hollow shapes, the brittle layers often crack or chip resulting in loss of the chuck. It is also difficult to precisely machine fine holes or grooves in the monocrystalline ceramic wafer.
Yet another problem with such conventional chucks arises from the method of fabrication of the monocrystalline ceramic wafers. In one method, the Czochralski-type method, large crystals of monocrystalline ceramic are drawn from molten alumina using a seed crystal mounted on a die. The drawn out material cools and solidifies to form a column of large and oriented crystals. Thereafter, the column is sliced to form monocrystalline ceramic wafers. Another method commonly known as the EFG process (edge-defined, film fed, growth process) is taught for example, by U.S. Pat. Nos. 3,701,636 and 3,915,662 to La Bella, et al., both of which are incorporated herein by reference. In these methods, a single crystal of monocrystalline ceramic is drawn from molten alumina, using a die such as an annular ring contacting the molten alumina in a capillary tube. The molten alumina rises in the tube via capillary forces and the die provides a seeding surface from which the monocrystalline ceramic crystal is grown. However, the size of the monocrystalline ceramic crystal grown by these methods is restricted by the dimensions of the size of the die opening, preventing growth of large monocrystalline ceramic crystals need for large diameter chucks. The fabrication methods can also produce crystals having relatively small grains and with facet defects. Also, the drawn out crystal can twist and turn during the drawing out process to provide a disoriented and faceted crystalline structure.
It is desirable to have a chuck made of monocrystalline ceramic that exhibits reduced thermal expansion mismatch, low rates of erosion in plasma environments, and reduced particulate generation during use in semiconductor processing. It is also desirable for the monocrystalline ceramic used in the chuck to provide stable and reliable electrical properties at high operating temperatures, preferably exceeding about 1000xc2x0 C. It is further desirable to have predefined shapes of grooves, slots and channels for holding cooling fluid or helium gas in the body of the chuck to regulate the temperatures of the substrate and chuck.
An electrostatic chuck of the present invention comprises monocrystalline ceramic material that exhibits reduced erosion and resistant particle generation, and provides stable electrical properties at high operating temperatures. The electrostatic chuck comprises a unitary monolith of monocrystalline ceramic having a receiving surface for receiving a substrate. An electrode is embedded in the unitary monolith for electrostatically holding the substrate upon application of a voltage thereto. An electrical connector extending through the unitary monolith is used to supply the voltage to operate the electrode.
The electrostatic chuck can be fabricated by solidification of molten ceramic or from a plurality of monocrystalline ceramic plates bonded to one another to form a monolithic structure. Preferably, the monocrystalline ceramic comprises large crystals having a diameter of about 0.5 to about 10 cm, and which are substantially oriented to one another in a single crystallographic direction. The electrode of the chuck can comprise a pattern of lattice defects induced in the ceramic plates, a pattern of dopant in the ceramic plates, or an electrode made of conducting metal. The monolithic chuck can be operated at elevated temperatures with little or no contamination of the substrate.
One method of forming the monocrystalline chuck comprises forming a plurality of monocrystalline ceramic plates comprising, for example, sapphire crystals substantially oriented to one another. An electrode is formed on one or more of the monocrystalline ceramic plates. The monocrystalline ceramic plates are bonded to one another to form a monolithic structure having the electrode embedded therein. The plates can be bonded to one another by applying a bonding compound comprising aluminum oxide to the monocrystalline ceramic plates and heat treating the bonding compound. Preferably, the bonding compound comprises an eutectic mixture of aluminum oxide and eutectic component, the eutectic mixture having a melting temperature of less than about 2000xc2x0 C.
In another method for forming the electrostatic chuck, ceramic material is melted in a mold to form molten ceramic. The mold has an internal shape of an electrostatic chuck. One or more of electrode forms, channel forms, and conduit forms are suspended in the molten ceramic, and a seeding crystal is maintained in contact with the molten ceramic. The molten ceramic is directionally cooled to form monocrystalline ceramic comprising large crystals substantially oriented to one another, and having the electrode forms, channel forms, or conduit forms embedded therein. One or more of the electrode, channel, or conduit shaping forms in the monocrystalline ceramic are then suitably treated, for example, in an oxidation heat treatment or wet chemical etching process, to form a unitary monolith of monocrystalline ceramic having an electrode, and channels or conduits for holding heat transfer fluid or gas, respectively. The method provides an intermediate product comprising a unitary monolithic monocrystalline ceramic having embedded therein one or more chemically erodible forms that are shaped to form channels and conduits in the unitary monolithic monocrystalline ceramic.
In another aspect useful for regulating the temperature of a substrate, the electrostatic chuck comprises a dielectric member having an electrode embedded therein, and a receiving surface for receiving the substrate. The dielectric member comprises a fluid conduit for circulating heat transfer fluid in the chuck. Preferably, the fluid conduit comprises first passageways at a distance D1 from the receiving surface, and second passageways at a distance D2 from the receiving surface, the distance D1 being greater than the distance D2. A fluid inlet supplies heat transfer fluid to the conduit and a fluid outlet removes the heat transfer fluid. The temperature of a substrate held on a receiving surface of the electrostatic chuck is regulated by supplying heat transfer fluid through first passageways that are at a distance D1 from the receiving surface, and removing the heat transfer fluid from second passageways that are at a distance D2 from the receiving surface. The distance D1 is sufficiently greater than the distance D2 to compensate for a rise in temperature, or cooling of, of the heat transfer fluid as it circulates through the chuck.